1. Multipath comparison of IEEE802.11g High Rate Proposals
May 2001
Multipath comparison of IEEE802.11g High Rate Proposals
Sean Coffey, Anuj Batra, Srikanth Gummadi, Chris Heegard, Matthew Shoemake
Texas Instruments
141 Stony Circle, Suite 130
Santa Rosa California
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Coffey et al, Texas Instruments

2. Contents CCK-OFDM is not 802.11a Receiver structures
May 2001
Contents
CCK-OFDM is not a
Receiver structures
Multipath performance comparisons
Conclusions
Coffey et al, Texas Instruments

3. What is wrong with CCK-OFDM that is right with pure 11a OFDM?
May 2001
What is wrong with CCK-OFDM that is right with pure 11a OFDM?
Coffey et al, Texas Instruments

4. Overhead and Data Payloads
May 2001
Overhead and Data Payloads
PBCC
11a
CCK-OFDM
No acks, 500 byte packets
Coffey et al, Texas Instruments

5. May 2001
Relative throughputs
Coffey et al, Texas Instruments

6. May 2001
The CCK-OFDM Dilemma
Any receiver requires overhead for channel estimation, tracking, etc:
802.11a is “pay as you go” - ultra-short preamble, 16 musecs
802.11b and PBCC-22 are “pay all up front” - 11b “short” preamble, 96 musecs
CCK-OFDM is “pay up front and again as you go” - 11b “short” preamble, plus (non-standard) OFDM preamble, 110 musecs
“Double the pain”
Coffey et al, Texas Instruments

7. Packet size & system performance
May 2001
Packet size & system performance
Compare performances – results are critically dependent on packet size
Short packets (e.g., MPEG-4 packets of 188 bytes) strongly favor “pay-as-you-go” approach  a/Hiperlan 2
Long packets increasingly favor “pay-all-up-front” approach”  PBCC-22 aimed at this application
Short or long, it won’t work well if it’s CCK-OFDM
Coffey et al, Texas Instruments

8. May 2001
PBCC-22 features
Excellent performance in full range of multipath conditions - much better than CCK-OFDM at comparable rates
This is the central technical point in dispute
Range advantage of PBCC 22 Mbps over CCK-OFDM 24 Mbps in multipath conditions is 30-40%.
These claims documented later; standard IEEE models were used.
Coffey et al, Texas Instruments

9. Multipath comparison of the proposals
May 2001
Multipath comparison of the proposals
First, for each proposal, assume same ground rules:
floating point implementation
full channel knowledge
standard IEEE multipath model
off-the-shelf algorithms
assume each uses receiver structure presented by proposers
Coffey et al, Texas Instruments

10. May 2001
PBCC-22 Receiver:
treat multipath and code as forming a composite state machine, or “super code”
decode the “super trellis” using any standard reduced state algorithm
Simulation results here assume whitened matched filter plus M-algorithm; standard material, very well understood:
whitened matched filter - Forney, 1972.
M-algorithm - Anderson, 1969.
Coffey et al, Texas Instruments

11. M-algorithm decoder background:
May 2001
M-algorithm decoder background:
M-algorithm operates like regular trellis decoder, but retains only best “M” paths at each depth
No restrictions on choice of M
straightforward way of trading performance versus complexity
natural receiver upgrade path
Main results use M = 64
we also present M = 8, M = 16, M = 32, M = 128.
Coffey et al, Texas Instruments

12. May 2001
M-algorithm decoder:
Assume the “state” consists of input data bits at last 8 time units
Compare last 4 time units to represent pure code state
Choice of 8 is arbitrary, other values possible
Assume each “state” remembers the full impact of the past on the future
Curve shown in Doc. 01/140 assumes instead that multipath is regenerated from last 8 time unit inputs
Coffey et al, Texas Instruments

13. Baseline comparisons, IEEE multipath model, 100 ns
May 2001
Baseline comparisons, IEEE multipath model, 100 ns
From Doc. 00/392r1
5 dB
Ideal channel knowledge, floating point implementations
Coffey et al, Texas Instruments

14. Baseline comparisons, 100 ns, contd.
May 2001
Baseline comparisons, 100 ns, contd.
From Doc. 00/392r1
Coffey et al, Texas Instruments

15. Implications of 5 dB advantage:
May 2001
Implications of 5 dB advantage:
5 dB translates to a factor of 3.1
For similar throughput and range, PBCC requires 3 times less received power than CCK-OFDM 24 Mbps – translates to greater battery life
For similar throughput and received power, PBCC has 40% more range than CCK-OFDM 24 Mbps
Assuming the “power of 3.3” model for path loss – this is the standard model used in (Doc /138r0)
Coffey et al, Texas Instruments

16. 100ns: 40% PBCC range advantage
May 2001
100ns: 40% PBCC range advantage
PBCC 22 Mbps
CCK-OFDM
24 Mbps
Double the coverage
Coffey et al, Texas Instruments

17. Relative throughputs:
May 2001
Relative throughputs:
Coffey et al, Texas Instruments

18. Actual PBCC receiver algorithms, 100 ns
May 2001
Actual PBCC receiver algorithms, 100 ns
2.9 dB
Ideal channel knowledge, floating point implementation for CCK-OFDM
Coffey et al, Texas Instruments

19. Baseline comparisons, contd: 250 ns
May 2001
Baseline comparisons, contd: 250 ns
4.5 dB
Ideal channel knowledge, floating point implementations
Coffey et al, Texas Instruments

20. Baseline comparisons, 250 ns, contd.
May 2001
Baseline comparisons, 250 ns, contd.
Coffey et al, Texas Instruments

21. May 2001
Conclusions
PBCC-22 has a natural superiority over CCK-OFDM in multipath.
Established IEEE multipath model used.
CCK-OFDM tries to juggle two incompatible things – makes an underperforming system out of a merger of two otherwise good components
A better way of doing things – PBCC a!
Coffey et al, Texas Instruments